- `override=True` doesn't work with our early-checking `Abstract`
- per-field frozen
- option to freeze after `__post_init__` / `dc.init`'s?
- transient field option
- derive - per-field and 'free'
- c codegen?
- alt backends?
  - persistent map
- omlish.metadata
- optional dirty tracking
- old ver had descriptor
  - default, frozen, name, pre_set, post_set
  - cext
- user specifiable arbitrary field descriptor?
- overhaul metaclass / 'confer'
- 